The invention relates to an improved process for removing dissolved metals from waste water using a recycle high density sludge. More specifically, the process uses a recycle bypass stream prior to the sludge separation step to reduce the solids loading in the sludge separation step. The process may also include a solids classification step to preferentially recycle the smaller solids particles precipitated from the waste water and discard the larger particles precipitated from the waste water.
The removal of dissolved metals from waste water streams is desired in many industrial applications. The dissolved metals may include iron, aluminum, magnesium, zinc, and manganese. Typically, the dissolved metals are present in the waste water as chlorides and sulfates. For example, iron may be present as ferrous chloride (FeCl.sub.2), ferric chloride (FeCl.sub.3), ferrous sulfate (FeSO.sub.4), and ferric sulfate [Fe.sub.2 (SO.sub.4).sub.3 ]. The chloride and sulfate salts of the dissolved metals create an acidic environment in the waste water due to the dissolution of the salts into ionic forms. Ferric chloride (FeCl.sub.3), for example, will dissolve in water to form trivalent iron ions (Fe.sup.3+) and chloride ions (Cl.sup.-).
Waste water containing dissolved metals occurs in numerous industrial processes. For example, acid mine drainage containing dissolved iron occurs as a result of mining operations. Known methods of treating waste water containing dissolved metals involve contacting the waste water with an alkaline material such as sodium hydroxide. The hydroxide compound causes the dissolved metals to precipitate as the corresponding metal hydroxide compounds. An example of this reaction is: EQU Fe.sub.2 (SO.sub.4).sub.3 +3 Ca(OH).sub.2 --2 Fe(OH).sub.3 +3 CaSO.sub.4
As noted above, the ferric sulfate [Fe.sub.2 (SO.sub.4).sub.3 ] forms trivalent iron ions (Fe.sup.3+) and sulfate ions (SO.sub.4.sup.2-) in the waste water. The ferric hydroxide [Fe(OH).sub.3 ] is generally insoluble and forms a precipitate. The metal hydroxide precipitate is separated from the water by a settling device such as a thickener. The settling device produces a sludge containing the settled material and a water effluent that is relatively free of solids and dissolved metals.
Known methods using this process, particularly Kostenbader (U.S. Pat. No. 3,738,932), have attempted to improve the process by recycling some portion of the precipitated sludge material. Referring to FIG. 1, a process flow diagram of the Kostenbader process is shown. A waste water stream 1 containing dissolved metals is contacted with recycle particles which include hydroxyl (OH.sup.-) groups in a precipitation reactor 2 to precipitate metal hydroxides on the surfaces of the recycle particles. The treated waste water stream 3 is fed to a separation device 4 which produces a water effluent stream 5 and a sludge stream 6. The water effluent stream 5 is relatively free of both dissolved metals and precipitated metal hydroxides.
A portion of the sludge stream 6 is discharged as waste sludge 8 and a portion is recycled as recycle sludge stream 7 to provide recycle particles for the precipitation reactor 2. The recycle sludge stream 7 is fed into adsorption reactor 9 along with alkaline reagent 10 where the alkaline reagent 10 forms hydroxyl ions (OH.sup.-) which are adsorbed onto the surfaces of the recycle particles in the recycle sludge stream 7. The stream 11 from the adsorption reactor 9 containing the recycle particles with adsorbed hydroxyl groups (OH.sup.-) is then fed to the precipitation reactor 2. It should be appreciated that any particular recycle particle starts out as a metal hydroxide precipitate particle which then continually grows due to layer upon layer of hydroxyl (OH.sup.-) ions then metal hydroxide precipitates which are added to the particle surface as it is continually recycled.
Typically, the separation device 4 is a thickener. Essentially, the thickener provides a large volume where the metal hydroxide precipitates which are denser than water will settle towards the bottom of the thickener due to gravity and leave a zone of relatively clear, solids-free water at the top of the thickener. The rate of settling of the metal hydroxide precipitates is dependent on multiple factors such as the density of the precipitates relative to the density of the water, the size of the precipitate particles, and the surface area of the thickener. It should be appreciated that the surface area required of a thickener will vary with the type and amount of precipitate particles being separated. Thus, an increase in the amount of precipitate particles to be settled normally requires an increase in the surface area of the thickener.
The Kostenbader patent teaches that a sludge stream preferably containing 20 to 30 pounds of precipitates is recycled for every pound of dissolved metals in the waste water feed stream. (Col. 5, lines 49-52; col. 3, lines 37-40). The primary benefit of the method of the Kostenbader patent is that it produces a high density sludge containing 15 to 50 weight percent solids as contrasted to other, nonrecycle methods which produce sludges of only 1 or 2 weight percent solids. (Col. 1, lines 42-44).
Because the recycle method of the Kostenbader patent uses a large amount of recycle solids for each pound of dissolved solids to be precipitated, it requires a thickener--the most commonly used separation device for this application--with a large surface area. This results in the disadvantage that when waste water feed streams have high concentrations of dissolved metals the amount of recycle solids must increase, and therefore the surface area of the thickener must be increased. Thus, the Kostenbader process may require a large number of thickeners or thickeners of impractical size when waste water feed streams have high concentrations of dissolved metals. Therefore, the Kostenbader process is generally ineffective or impractical for removing dissolved metals from waste water streams with high concentrations of dissolved metals.